1. Field of the Invention
This invention relates to high impedance structures that allow microstrip antennas to radiate at more than one frequency and waveguides to transmit at more than one frequency.
2. Description of the Related Art
Microstrip patch and strip antennas are often used in applications requiring a low profile, light weight and bandwidths less than a few percent. The basic microstrip antenna includes a microstrip line resonator consisting of a thin metallic conducting patch etched on a dielectric substrate and conductive layer on the dielectric substrate's surface opposite the resonator. [CRC Press, The Electrical Engineering Handbook 2nd Edition, Dorf, Pg. 970, (1997)] The dielectric substrate is commonly made of TEFLON→fiberglass that allows it to be curved to conform to the shape of the mounting surface, and the conductive materials are commonly made of copper. The substrate generally has a thickness approximately equal to one fourth of the wavelength of the antenna's radiating signal. This provides the electrical distance between the conductive layer and antenna's radiating element to promote signal radiation into one hemisphere and to provide optimal gain.
One disadvantage of these types of antenna is that the fixed electrical distance between the radiating element and the conductive layer limits efficient radiation to a narrow bandwidth around a center frequency. The radiation and other related properties (antenna impedance, for example) will be seriously degraded as the operating frequency moves away from the center frequency. Another disadvantage of this structure is that the dielectric substrate and the conductive layer can support surface and substrate modes that can further degrade antenna performance. Also, surface currents can flow on the conductive layer that can deteriorate the antenna pattern by decreasing the front-to-back ratio.
A photonic surface structure has been developed which exhibits a high wave impedance to a signal's electric (E) field over a limited bandwidth. [D. Sievenpiper, “High Impedance Electromagnetic Surfaces,” (1999) PhD Thesis, University of California, Los Angeles]. The surface structure comprises “patches” of conductive material mounted in a substrate of dielectric material, with “vias” of conducting material running from each patch to a continuous conductive sheet on the opposite side of the dielectric substrate. The structure appears similar to numerous thumbtacks through the substrate to the conductive sheet. It presents a series of resonant L-C circuits to an incident E field of a specific frequency, while the gaps between the patches block surface current flow.
This structure can be used as the substrate in a microstrip antenna to enhance performance by suppressing the antenna surface and substrate modes. It also increases the front to back ratio by blocking surface current. However, it only functions within a small bandwidth around a center frequency. As the frequency moves from the center, the structure will appear as a conductive plane that can again support undesirable modes.
New generations of communications, surveillance and radar equipment require substantial power from solid state amplifiers at frequencies above 30 gigahertz (GHz). Higher frequency signals can carry more information (bandwidth), allow for smaller antennas with very high gain, and provide radar with improved resolution. For solid state amplifiers, as the frequency of the signal increases, the size of the transistors within the amplifiers and the amplifier power output decrease. At higher frequencies, more amplifiers are required to achieve the necessary power level. To attain power on the order of watts, for signals having a frequency of approximately 30 GHz, hundreds of amplifiers must be combined. This cannot be done by power combining networks because of the insertion loss of the network transmission lines. As the number of amplifiers increases, a point will be reached at which the loss experienced by the transmission lines will exceed the gain produced by the amplifiers.
One current method of amplifying high frequency signals is to combine the power output of many small amplifiers oriented in space in a two-dimensional quasi-optic amplifier array. The array amplifies a beam of energy normal to it rather than a signal guided by a transmission line. It can combine the output power of hundreds of solid state amplifiers within the array. A waveguide can guide the beam of energy to the array, or the beam can be a Gaussian beam aimed in free space at the array. [C. M. Liu et al., Monolithic 40 Ghz 670 mW HBT Grid Amplifier, (1996) IEEE MTT-S,p.1123].
One type of waveguide for high frequency signals has a rectangular cross-section and conductive sidewalls. A signal source at one end transmits a signal down the waveguide to a quasi-optical amplifier array mounted at the opposite end, normal to the waveguide. However, this type of waveguide does not provide an optimal signal to drive an amplifier array. For instance, a vertically polarized signal has a vertical electric field component(E) and a perpendicular magnetic field component(H). Because the waveguides sidewalls are conductive, they present a short circuit to the E field, which therefore must be zero at the sidewalls. The power densities of both the E and H fields drop off as the sidewall is approached. The power density of the transmission signal varies from a maximum at the middle of the waveguide to zero at its sidewalls. If the waveguide's cross-section were shaped to support a horizontally oriented signal, the same problem would exist with the signal dropping off near the waveguide's top and bottom walls.
This power drop-off reduces the amplifying efficiency of the amplifier array. For efficient amplification, each individual amplifier in the array should be driven by the same power level, i.e. the power density should be uniform across the array. When amplifying the type of signal provided by a metal waveguide, the amplifiers at the center of the array will be overdriven before the edge amplifiers can be driven adequately. Also, individual amplifiers in the array will see different source and load impedances, depending upon their locations in the array. The reduced power amplitude, along with impedance mismatches at the input and output, make most of the edge amplifiers ineffective. The net result is a significant reduction in the potential output power.
Waveguides having high impedance walls can transmit a signal without the E and the H fields dropping off at the sidewalls. For example, with the Sievenpiper thumbtack high impedance surface (described above) on the sidewalls and with the waveguide transmitting a vertically polarized signal, the sidewalls will appear as an open circuit to the signal's E field. The E field will be transverse to the sidewalls and will not experience the drop-off associated with a conductive surface. Current will also flow down the waveguide's top and bottom walls to support a uniform H field. However, because the gaps between the patches of the high impedance structure do not allow surface conduction in any direction, the waveguide cannot transmit cross-polarized signals with uniform density. Also, the waveguide can only transmit a signal within a limited bandwidth of the center frequency.
A high impedance wall structure has also been developed having conductive strips instead of conductive patches. [M. Kim et al., A Rectangular TEM Waveguide with Photonic Crystal Walls for Excitation of Quasi-Optic Amplifiers, (1999) IEEE MTT-S, Archived on CDROM]. The wall is particularly applicable to rectangular waveguides transmitting cross-polarized signals. Either two or four of the waveguide's walls can have this structure, depending upon the polarizations of the signal being transmitted. The wall comprises a substrate of dielectric material with parallel strips of conductive material that are equal distances apart. It also includes conductive vias through the sheet to a conductive sheet on the substrate's surface opposite the strips. When used for the walls of a rectangular waveguide, the structure provides a high impedance termination for the E field component of a signal and also allows conduction through the strips to support the H field component. When used for all four of the waveguide's walls, the waveguide can transmit cross-polarized signals similar to a free-space wave having a near-uniform power density.
However, like the thumbtack structure, the strip structure only functions within a limited bandwidth of a center frequency. Outside the bandwidth the wall will appear as a conductive surface to the signal, and the power densities of the E and H fields will drop off towards the waveguide's walls. The waveguide can efficiently drive an amplifier array only within a small bandwidth around a specific center frequency.
Dielectric-loaded waveguides, so called hard-wall horns, have been shown to improve the uniformity of signal power density. [M. A. Ali, et.al., Analysis and Measurement of Hard Horn Feeds for the Excitation of quasi-Optical Amplifiers, (1998) IEEE MTT-S, pp. 1913-1921]. While an improvement in uniformity, this approach still does not provide optimal performance for an amplifier array in which input and output fields of a signal are cross polarized.